The inverse square law for illuminance is valid for:

An electric lamp is fixed at the ceiling of a circular tunnel as shown in the figure. What is the ratio of the intensities of light at base $A$ and a point $B$ on the wall?

Two lamps of luminous intensity of $8 \, Cd$ and $32 \, Cd$ respectively are lying at a distance of $1.2 \, m$ from each other. Where should a screen be placed between the two lamps such that its two faces are equally illuminated due to the two sources?

For a candle of $40$ power,the exposure time required for a photographic print at a distance of $0.6 \, m$ is $20 \, s$. What will be the exposure time required for the same print at a distance of $1.2 \, m$ using a candle of $20$ power?

The curvature radii of a concavo-convex glass lens are $20\, cm$ and $60\, cm$. The convex surface of the lens is silvered. With the lens horizontal,the concave surface is filled with water. The focal length of the effective mirror is $(\mu_{\text{glass}} = 1.5, \mu_{\text{water}} = 4/3)$.

In a movie hall,the distance between the projector and the screen is increased by $1\%$. The illumination on the screen is: